Mirrored from Sudopedia, the Free Sudoku Reference Guide
In a Killer Sudoku, an outie is a cell that belongs to one of the selected cages, but lies outside the tested house(s). It is used in the 45 rule, a solving technique used for Killer Sudokus. The digit for a single outie can be calculated as follows:
(sum of selected cages) - (number of tested houses) x 45
The green cell r2c6 is an outie for nonet 3. Its value is 13 + 15 + 18 - 45 = 1.